The increase in bandwidth demand for optical links, such as the links found in long haul optical networks, can be satisfied by increasing the capacity of optical link. Increasing the baud rate is among the methods that can be used to increase the capacity of the optical links. Increasing the baud rate requires an increase in the clocking speed of the Digital Signal Processing (DSP) chip, except if the data samples are processed in bigger block sizes (parallelizing the data path). There is also demand for energy consumption reduction in the DSP chip, which may be obtained by reducing clock speed and increasing DSP processor block size. The drawback of increasing the DSP processor block size is the discontinuity in the adaptation of the DSP loops. It can be challenging to retain the performance of the DSP algorithms while reducing the loops update rate.
Coherent optical receivers typically use adaptive equalizers to compensate for time-varying polarization, mode dispersion effects, and inter-symbol interference (ISI). Performance of coherent optical receivers often relies heavily on carrier recovery (CR) methods. The interplay between adaptive equalization and CR has led to two types of coherent receiver architectures: blind equalization with independent CR; and joint equalization and CR.
For blind equalization with independent CR, the adaptive equalizer can use a constant modulus algorithm (CMA) for equalizer convergence. In this case the equalizer is decoupled from the carrier recovery and potentially has low complexity. However, the CMA typically exhibits a slow convergence rate, and this scheme can be sensitive to higher order modulations, such as 16QAM or higher.
For joint equalization and CR, the equalizer can be adapted in a decision directed (DD) manner, for example using a Least Mean Squares (LMS) algorithm. This approach exhibits faster convergence and better performance than blind equalizers. However, for implementation, the equalizer typically requires full knowledge of signal phase offsets to update its filter coefficients. Furthermore, due to its limited speed, the presence of the CR operation the adaptive equalizer feedback loop limits the speed of equalizer adaptation.
Therefore there is a need for an adaptive equalizer in a signal receiver, for example in an optical modem, that is not subject to one or more limitations of the prior art, such as decreasing performance with increasing data block size.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.